One method of forming a thin sheet of glass is by a drawing process where a ribbon of glass is drawn from a reservoir of molten glass. This may be accomplished, for example, via an up-draw process, where the ribbon is drawn upward from the reservoir (e.g. Foucault or Colburn), or by a down-draw process (e.g. slot or fusion), where the ribbon is drawn downward, typically from a forming body. Once the ribbon is formed, individual sheets of glass are cut from the ribbon.
In a conventional downdraw process, such as a fusion downdraw process, the molten glass is formed into a glass ribbon contained within a draw chamber defined by a shroud that surrounds the ribbon. Among other things the shroud serves to maintain a consistent thermal environment in the region defined by the shroud and surrounding the ribbon. Roller pairs penetrate the shroud and pinch the ribbon edges. The rollers (or rolls) may be used to apply a pulling force to the ribbon, to apply a transverse tension to the ribbon, or merely to guide the ribbon. Accordingly, a rotational force may be applied to the rolls by a motor, or the rolls may be freewheeling and the rotational force applied to the rollers by the descending ribbon. In either case, the rolls rotate. Production roll mechanisms typically allow for the rolls to move horizontally and/or vertically from the glass contact area. This accommodates geometric tolerances of the rolls, run out and tolerance changes in operation, along with normal variability in glass thickness. Further, production roll mechanisms typically allow the rolls to move far away from the glass for maintenance access, process restart, and other practical considerations. However, the rolls must also accommodate thickness changes at the edges of the ribbon, or dimensional variations in the edge rollers themselves. Frictional forces that resist the free motion of the edge rolls traverse to the ribbon during production operation may induce force cycling that manifests as undesirable perturbations or stress changes in the ribbon that can become frozen into the glass as the glass transitions from a viscous material to an elastic material.